This invention generally relates to refractory metal intermetallic composites. Some specific embodiments of the invention are directed to methods for casting such composites and providing them with protective coatings.
Turbines and other types of high-performance equipment are designed to operate in a very demanding environment which always includes high-temperature exposure, and often includes high stress and high pressure. Superalloys based on elements like nickel or cobalt have often provided the chemical and physical properties required for such operating conditions.
While the attributes of superalloys continue to ensure considerable interest in such materials, new compositions have been developed to meet an ever-increasing threshold for high-temperature exposure. Prominent among such materials are the refractory metal intermetallic composites (RMIC's). Examples include various niobium-silicide alloys. (The RMIC materials may also include a variety of other elements, such as titanium, hafnium, aluminum, and chromium). These materials generally have much greater temperature capabilities than the current class of superalloys. As an illustration, while many nickel-based superalloys have an operating temperature limit of about 1100° C., many RMIC alloys have an operating temperature in the range of about 1200° C.-1700° C. These temperature capabilities provide tremendous opportunities for future applications of the RMIC alloys. Moreover, the alloys are considerably lighter than many of the nickel-based superalloys.
Clearly, RMIC alloys possess very attractive properties which make the materials desirable for many demanding applications. However, continued improvement in some areas would be welcome in the art. One area of need for some embodiments relates to environmental protection, e.g., resistance to oxidation and corrosion. For example, at temperatures above about 1090° C., some of the Nb-based RMIC alloys can undergo rapid oxidation. While a slow-growing oxide scale can form on the alloys at this temperature, it is not typically a protective oxide scale. Moreover, another type of undesirable oxidation known as “pesting” can sometimes occur at intermediate temperatures, e.g., in the range of about 600° C.-980° C. (1112° F.-1800° F.). Refractory metals, particularly molybdenum, sometimes exhibit relatively low resistance to pesting oxidation.
In order to increase oxidation resistance through various temperature ranges, a number of protective coatings for RMIC articles have been developed. Many of them are described in U.S. Pat. Nos. 5,721,061 (Jackson and Ritter) and 6,497,968 (Zhao, Bewlay, and Jackson). Most of these coatings are silicon-based, and also include chromium and titanium. Most (though not all) of the coatings also include niobium, along with other optional elements. When applied on a cast RMIC article and heat-treated, the protective coatings often contain a chromium-rich phase.
While these protective coatings are generally effective in minimizing the problems of oxidation, their deposition and formation can sometimes be time-consuming and difficult. As an example, the coating constituents often have to be carefully pre-mixed in a suitable slurry which then must be carefully sprayed or painted onto the RMIC article. Heat treatments must often be undertaken to cure the coating and promote reaction of the coating constituents with each other, and with the RMIC surface. Moreover, the application of the protective coatings to various cavities and apertures can be difficult and incomplete. For example, turbine blades made from RMIC alloys usually include radial cooling holes or serpentine passageways which can extend entirely through the part. It can be very difficult to physically apply an adherent protective coating through such a length.
RMIC materials, like nickel and cobalt superalloys, can be cast into useful articles by various techniques. One of the most popular techniques is investment casting, sometimes referred to as the “lost wax process”. The overall process usually begins with the construction of a shell mold. Typically, a wax model is dipped into a slurry comprising a binder and a refractory material, so as to coat the model with a layer of slurry. (The binder is often a silica-based material). Additional layers of dry refractory material and stucco-slurry layers are applied as appropriate, to form a shell mold around the wax model, having a suitable thickness. After thorough drying, the wax model is eliminated from the shell mold, and the mold is fired. In the actual casting step, molten material for the desired alloy is introduced into the shell mold by various conventional techniques. The molten material is then cooled to form a solid cast article. In many instances, one or more cores are incorporated into the shell mold, to provide the various holes and passageways described previously. The core material is later removed from the final casting by conventional techniques.
While shell mold structures are very suitable for casting RMIC alloys in many situations, some serious drawbacks and other considerations are associated with their use. For example, niobium-silicides are chemically-reactive materials which can react with the silica in the wall of a shell mold. This type of reaction can result in serious surface defects in the cast article. These defects can limit precision casting. In some cases, over-size parts must be cast and then machined-to-size in order to remove the surface defects.
Many of these drawbacks are addressed by the use of facecoats, which form a protective barrier between the molten RMIC casting metal and the surface of the shell mold. For example, U.S. Pat. No. 6,676,381 (Subramanian et al) describes a facecoat based on yttria or at least one rare earth metal and other inorganic components, such as oxides, silicides, silicates, and sulfides. The facecoat compositions are most often in the form of a slurry which includes a binder material, along with a refractory material like the yttria component. When a molten, reactive casting metal is delivered into the shell mold, the facecoat prevents the undesirable reaction between the casting metal and the walls of the mold, i.e., the walls underneath the facecoat. Facecoats can sometimes be used, for the same purpose, to protect the portion of a core within the shell mold, which would normally come into contact with the casting metal. While facecoats are very effective for these purposes, their use requires additional materials and process steps for the overall manufacturing process.
In view of the various considerations set forth above, it should be apparent that additional advancements in RMIC technology would be welcome in the art. Improvements directed to the casting processes for RMIC alloys would be of substantial value. Improvements for applying protective coatings to both the external and internal surfaces of the cast articles would also be of special interest. Any increase in casting and coating efficiency would be very beneficial in commercial production facilities.